The production of steel normally comprises the steps of converting iron ore to pig iron using a blast furnace, and thereafter converting the pig iron into steel using an open hearth furnace or a converter. Such a traditional method requires large amounts of energy and large-scale equipment, and has a high cost. Therefore, for a small-scale steel-making, a method comprising the steps of directly converting iron ore into raw materials used in the steel-making furnace, and converting the raw material into steel using an electric furnace and the like has been used. With respect to direct steel making process, a direct reduction process has been used to convert iron ore into reduced iron. However, the reduced iron produced by the direct reduction process is highly reactive and reacts with oxygen in the air to generate heat. Therefore, it is necessary to seal the reduced iron with an inert gas, or by some other measures, during transportation and storage of the reduced iron. Accordingly, iron carbide (Fe3C) containing a comparatively high iron (Fe) content, which has a low reaction activity and can be easily transported and stored, has recently been used as the iron-containing material for steel making in an electric furnace and the like.
Furthermore, an iron-making or steel-making material containing iron carbide as the main component is not only easy to be transported and stored, but also has the advantage that the carbon combined with iron element can be used as a source of fuel in an iron-making or steel-making furnace, and can be used as a source to generate microbubbles which accelerate the reaction in the steel-making furnace. Therefore, raw materials for iron making or steel making containing iron carbide as the main component recently have attracted special interest.
According to a conventional method for producing iron carbide, fine-sized iron ore is charged into a fluidized bed reactor or the like, and is caused to react with a gas mixture comprising a reducing gas (e. g., hydrogen gas) and a carburizing gas (e.g., methane gas and the like) at a predetermined temperature. Thus, iron oxides (e. g., hematite (Fe2O3), magnetite (Fe3O4), wustite (FeO)) in iron ore are reduced and carburized in a single process (which means a process performed by simultaneously introducing a reducing gas and a carburizing gas to a single reactor). The prior art in the field of the present invention has been described, for example, in the publication No. 6-501983 of the Japanese translation of International Patent Application (PCT/US91/05198).
The iron carbide producing process can be expressed by the following general reaction formula. EQU 3Fe2O3+5H2+2CH4.fwdarw.2Fe3C+9H2O
In the single process, however, reducing reaction and carburizing reaction should be taken into consideration together. In addition, a reaction gas composition and a reaction gas temperature which are suitable for each reaction cannot be applied. As a result, a reaction time (which is required for conversion into iron carbide) becomes longer. As compared with a conventional method, it takes a longer time to obtain a constant amount of raw materials for steel making. For this reason, there is a drawback that an equipment scale should be enlarged to increase the production per unit time.
The present inventors have filed a patent application Japanese Patent Application No. 8-30985) related to novel technology for a method and apparatus for producing iron carbide which can perform various actions for each operation, increase flexibility as a process, shorten a reaction time and reduce an amount of a reaction gas to be used. This invention relates to a method for producing iron carbide comprising the steps of performing a first-stage reaction process for carrying out a part of reducing reaction of iron ore comprising hematite as the main component and then performing a second-stage reaction process for carrying out further reducing and carburizing reaction and has eliminated all drawbacks of the conventional iron carbide producing method and is an epoch-making method for producing iron carbide.
However, also in the case where iron carbide is produced in the two-stages reaction process, an iron carbide product having a goal composition cannot always be obtained.
The reason is as follows. A lot of reaction parameters such as a reaction gas composition, a reaction temperature, a reaction pressure and the like are concerned in generation of the iron carbide. In some cases, the reaction parameters are slightly changed so that undesired products (having a low rate of conversion into iron carbide, for example) are obtained. If the reaction parameters get out of a constant range, free carbon is sometimes generated.
There has been proposed a method for controlling quality of an iron carbide product characterized in that whether a composition of an obtained product can be permitted or not is checked by the Mbssbauer analysis method in order to control a composition of an iron carbide product within a constant range, and the reaction parameters are changed if the composition is not kept within a tolerance. (For example, see U.S. Pat. No. 5073194, PCT/US91/05188).
However, the Mossbauer analyzer has a drawback that it takes a long time (1 to 4 hours) to perform the measurement in order to enhance the precision. Accordingly, there has been a problem that it is impossible to take the proper actions corresponding to conditions in a reactor which are changed momently.
In consideration of the above-mentioned problems of the prior art, it is an object of the present invention to provide a method for managing an operation of a producing process for obtaining an iron carbide product having a goal composition in a two-stages reaction process.